--- layout: default title: Abstract Syntax Tree description: Using the Abstract Syntax Tree (AST) to manipulate the parsed content --- # Abstract Syntax Tree This library uses a doubly-linked list Abstract Syntax Tree (AST) to represent the parsed block and inline elements. All such elements extend from the `Node` class. ## Traversal The following methods can be used to traverse the AST: - `previous()` - `next()` - `parent()` - `firstChild()` - `lastChild()` - `children()` ## Iteration / Walking the Tree If you'd like to iterate through all the nodes, use the `walker()` method to obtain an instance of `NodeWalker`. This will walk through the entire tree, emitting `NodeWalkerEvent`s along the way. ```php use League\CommonMark\Node\NodeWalker; /** @var NodeWalker $walker */ $walker = $document->walker(); while ($event = $walker->next()) { echo 'I am ' . ($event->isEntering() ? 'entering' : 'leaving') . ' a ' . get_class($event->getNode()) . ' node' . "\n"; } ``` This walker doesn't use recursion, so you won't blow the stack when working with deeply-nested nodes. ## Modification The following methods can be used to modify the AST: - `insertAfter(Node $sibling)` - `insertBefore(Node $sibling)` - `replaceWith(Node $replacement)` - `detach()` - `appendChild(Node $child)` - `prependChild(Node $child)` - `detachChildren()` - `replaceChildren(Node[] $children)` ## `DocumentParsedEvent` The best way to access and manipulate the AST is by adding an [event listener](/1.3/customization/event-dispatcher/) for the `DocumentParsedEvent`.